Fluid warming systems designed to warm parenteral fluids and blood products (hereinafter “fluids”) for infusion into a patient are in common use. Generally, such systems include a warming unit and a flow path device constructed to operate cooperatively with the warming unit by conducting fluid through a flow path in a heating region of the warming unit where the heat is transferred to the fluid as it flows. For example, parenteral fluid warming equipment may include a conductive warming unit and a fluid warming cassette that may be removably received in the warming unit. The fluid warming cassette typically includes a fluid container with a structure designed for being received and supported in the warming unit. Such a fluid container consists of sheets of plastic film material and/or thin metal foil joined, usually by heat or adhesives, to define a fluid channel. Inlet and outlet ports are provided in the fluid channel to receive tubing through which fluid flows into and out of the channel.
When such a fluid warming system is put to use with the fluid warming cassette placed or positioned in the heating region, heat is transferred from the warming unit to and through the cassette to heat fluid as it flows through the fluid channel. In the heating region, heat is transferred by one or more modes including conduction, convection, and radiation. Typically a warming unit is designed for a principal mode of heat transfer to the external surfaces of the cassette. The cassette is constructed for transferring heat to the fluid by conduction from its external surfaces through the layers of the fluid container. One example of a warming unit designed for heat transfer by conduction includes metal plates and means for electrically warming the plates. The metal plates are positioned in an opposing disposition for close frictional contact with one or more surfaces of a cassette. Typically, the plates are slightly separated to define a thin slot into which the cassette may be slid. When the cassette is positioned in contact with the plates while the plates are warmed, heat flows from the plates to the cassette surfaces and through the cassette to the fluid channel, thereby heating fluid as it flows through the channel. To maximize the thermal efficiency and thermal responsiveness of a fluid warming system with a slotted warming unit in which a fluid warming cassette is disposed for conductive heat transfer from the warming unit, the distance between the plates is usually kept very small. This necessitates a fluid warming cassette with a thin, flat fluid container. One such cassette is disclosed in U.S. patent application Ser. No. 09/415,558, entitled “PRESSURE TOLERANT PARENTERAL FLUID AND BLOOD CONTAINER FOR A WARMING CASSETTE”, by Augustine et al., filed on Oct. 8, 1999, which is incorporated herein by this reference.
A number of design parameters are important to maximizing the thermal conductivity at the interface between the plates of a conductive warming unit and the fluid warming cassette. For example, very thin films of thermally conductive plastic materials are typically used to reduce the thickness of the container and the length of the thermal conduction path through the container to the fluid channel. A design goal is to maximize the total external surface area of the fluid container which contacts the plates in order to maximize heat transfer to the container, and to invest the structure of the container with the ability to maintain that surface area in contact with the plates in the face of variations in the pressure of fluid flow. This leads to the selection of plastic sheets formed from relatively rigid plastic materials. In this regard, a rigid plastic is as defined in Whittington's Dictionary of Plastics, Third Edition, as one with a modulus of elasticity either in flexure or in tension greater than 700 MPa (100 kpsi) at 23° C. and 50% relative humidity when tested in accordance with ASTM methods D747, D790, D638, or D882 (ASTM D833). The same definition gives other specifications for rigid vinyl.
In use, such a fluid container is operated by provision of fluid under positive pressure to its inlet port, which causes the fluid to flow through the container and keeps the fluid channel open. The pressure is positive with respect to ambient pressure, and is usually provided either by a fluid reservoir elevated above the fluid container, or by an infusion pump. When deployed for pediatric cases, in combination with a pressure cuff presently-available cassettes may quickly infuse a large amount of fluid into a small patient, causing undesirable effects and, possibly, harm. One way to limit the volume which may be delivered to an infant or child is to limit the amount of fluid delivered at some maximum pressure by limiting or reducing the cross-sectional dimensions of the fluid flow path of a cassette. This, however can lead to other problems in other circumstances.
It is frequently useful to apply a negative pressure through the outlet side of the fluid warmer cassette in order to draw fluid through the fluid channel. Such negative pressure may be applied, for example, with a syringe coupled to the outlet port through a three-way valve and a piece of tubing. This configuration is used in cases where fluid must be cleared from the cassette, and in cases where a bolus of warmed fluid is to be drawn through the cassette, into the syringe. Negative pressure however interacts adversely with certain structural features of presently-available cassettes. Cassettes made by welding thin films of rigid plastic over rigid spacers exhibit collapse of their fluid channels in response to negative pressure. The collapse is usually profound: it extends along the entire length of the fluid channel.
One way to reduce the tendency of the fluid channel to collapse in response to negative pressure is to increase the thickness and rigidity of the film layers of which the fluid container is constructed. However, the thicker, rigid materials significantly increase heat transfer impedance. During the manufacturing and assembly processes the thicker, rigid materials also result in increased dimensional tolerances, which lead to reduction in contact between these materials and the warming plates caused by material and hardware tolerances. Furthermore, it is difficult to make a fluid channel from plastic films that are altogether resistant to negative pressure, and any bowing or partial collapse of a fluid channel under negative pressure will further reduce surface contact between the film layers and the warming plates.
The application of negative pressure to a warming cassette fluid container made of rigid plastic will cause some degree of contraction along the entire length of the fluid flow path. Unless the rigid fluid container is evenly preloaded against the plates of a warming unit, this contraction will pull the surface of the fluid container away from the plates precisely when warming is required, that is when fluid is being drawn through the fluid path. It is possible to make the fluid container slightly oversized with respect to the slot between the plates, which will preload it against the plates. But this produces difficulty in inserting the cassette between the plates. In some instances, warming units are made with separable plates that can be clamped onto a warming cassette. However, such mechanisms are costly and require a higher incidence of maintenance than mechanisms with fixed plates.
Manifestly, then, there is a need for an effective fluid warming cassette useful in a parenteral fluid warming system in which fluid continues to flow when the cassette is received in a slot between warming unit plates and negative pressure is applied to the outlet port for priming the fluid path or for drawing a bolus. It would be further advantageous if the fluid flowing through the fluid warming cassette in response to this negative pressure would also be warmed. Additional advantage would be gained if the priming volume of the fluid warming cassette were such that the flow path could be primed with a standard syringe.
It would also be advantageous if the fluid warming cassette could be designed for insertion between close-set parallel plates of a warming unit, yet be thin enough to efficiently transfer heat by conduction from the plates to the fluid during negative pressure.